The image luminance contrast (difference in luminance) generated in an MR image is controllable by the application of radio-frequency (RF) pulses. One class of RF pulse is the inversion recovery (IR) pulse which tips the equilibrium magnetization aligned along the main magnetic field (+z direction) to the −z direction. IR pulses have been used to produce T1 weighting, allowing for the differentiation of tissues based on their T1 values. A specific example of this application of IR pulses is known as delayed-enhancement MRI (and also known as late gadolinium-enhancement [LGE] imaging), which enhances the image luminance contrast between normal and infarcted (or scarred) myocardium. A known variant of delayed-enhancement imaging is called phase sensitive inversion recovery (PSIR) imaging which acquires T1-weighted inversion recovery images and reconstructs them with a phase sensitive reconstruction. With PSIR reconstruction, the sign of the magnetization in the anatomical MR image is retained, causing the image luminance contrast between normal and infarcted myocardium to be less sensitive to the inversion time (TI) used.
A known PSIR data acquisition is shown in FIG. 1. A non-selective IR pulse 203 causes magnetization 205 to rotate from the +z-axis to the −z-axis. An anatomical dataset acquisition at time 207 is followed by a reference dataset acquisition at time 209 being acquired without a preceding IR pulse at a later time, about TR after the anatomical dataset acquisition (for example one heartbeat later). A resulting PSIR image is obtained by correcting the phase of the anatomical dataset by using the phase of the reference dataset. In the resulting phase corrected anatomical image, the image intensity scale is automatically adjusted such that the smallest signal which is typically zero or negative appears black, and the most positive appears bright.
Fat suppression in MRI is used to ensure high quality images in regions with significant fat content. A variety of fat suppression methods are known. Frequency selective saturation pulses can be used to selectively suppress fat signal. One known example is a chemical shift selective (CHESS) method in which a saturation pulse is played at the fat resonance frequency followed by a spoiling gradient to destroy fat magnetization. While this method is effective for spectroscopy applications, the data acquisition time typically required for clinical imaging (approximately 100-300 ms) is so long that a large amount of the fat magnetization has recovered from the saturation pulse resulting in poor fat suppression capability.
Another known method of fat suppression is the short tau inversion recovery (STIR) pulse sequence. This method is used in connection with turbo-spin echo (TSE) readout and dark-blood (DB) preparation. A non-frequency selective but usually spatially-selective IR (NFSIR) pulse is timed to null fat signal at the beginning of the TSE readout and not the center of k-space. STIR suppresses fat signal well due to the nature of the TSE readout. The first pulse of a TSE readout train is a 90 degrees pulse that “locks in” the nulled fat signal. In response to the pulse, the longitudinal relaxation of fat is irrelevant for the remainder of the readout. Gradient echo (GRE, Siemens proprietary name Flash, fast low angle shot) and steady state free precession (SSFP, Siemens proprietary name TrueFisp, true fast imaging with steady precession) readouts do not have this “lock-in” property and thus require different timing between an NFSIR (Non-frequency Selective Inversion recovery) pulse and the beginning of the dataset acquisition. Such timing restricts the maximum number of lines in a dataset that can be acquired after an NFSIR pulse, often below a clinically useful value. Thus, in practice the STIR sequence works best in combination with TSE readout. Furthermore, STIR works only with a single inversion time which is used to null fat signal. It is substantially not feasible to apply an additional non-frequency selective IR pulse to impart T1-contrast, as the application of both pulses unfavorably alters image luminance contrast and prevents suppression of fat signal. In addition, to avoid image artifacts dark blood (DB) preparation is required to be used with TSE readout and consequently with the STIR method. DB preparation needs to be used in the absence of contrast agent due to timing limitations. Therefore, STIR may only be used without contrast agent.
Another known method of fat suppression method, known SPAIR (Spectral Selection Attenuated Inversion Recovery) or known SPIR (Spectral Presaturation Inversion Recovery) pulse provide fat suppression. These methods work in a similar way to STIR with a difference being that a non-frequency selective IR pulse is replaced by a SPAIR or a SPIR pulse. Both pulses are fat-frequency selective and spatially non-selective. The problems are similar to those of STIR, but both pulses can be used as a fat-frequency selective inversion pulse.
In addition to the frequency selective methods described above, another class of known fat suppression methods recognizes that due to the differences in frequency between fat and water, their signals will go in and out of phase. The classic application of this is the Dixon method. By acquiring two images with different echo times (TEs), one where water and fat are in phase and another where they are out of phase, the Dixon method allows the user to add or subtract the images to create water or fat only images. A limitation of this method is that the quality of the fat suppression is highly sensitive to inhomogeneity in the static magnetic field B0. In order to overcome this sensitivity to magnetic field inhomogeneity, often several (more than 2) images are acquired with different TEs, increasing scan time, the specific absorption rate (SAR, the rate at which RF energy is absorbed) and reconstruction time.